For many thermostats, such as automatic shut-off controls to prevent circuit overheating, a bimetal blade thermostat is used. This type of device usually includes an outer case and an internal thermostat blade which is formed of a bi-layer metal, each layer having a different coefficient of thermal expansion. The blade is usually attached at one end to one portion of the case. Upon heating the thermostat above a predetermined temperature, the free end of the blade will bend toward the layer with a lower coefficient and that end of the blade will move away from a contact point, breaking the electrical connection.
Thermostats such as these have applications in microelectronic devices, where space is always a serious concern. For example, in cellular phones, these thermostats can be used to disconnect the battery if it begins to overheat. Obviously, since some cellular phones can entirely fit in a pocket, space for the internal components comes at a high premium. This type of thermostat can also be used to switch a battery charging circuit between a fast charging current and a trickle current based on the temperature of the battery. Common thermostat devices have a case with a rectangular cup. The top edge of the cup extends outwardly to form a flange. To seal the case, a layer of insulation and then a conductive cover are placed on top of the cup. The flange, along with the insulation are then folded up and over the cover and crimped, sealing the blade within the case, the insulation separating the flange from the cover. This type of thermostat is disclosed in, for example, U.S. Pat. No. 4,136,323.
However, this type of known thermostat case requires a thickness dictated by the thickness of the case, insulating and cover materials. As can be seen in prior art FIG. 1, the minimum thickness is determined by the depth of the cup plus five layers of material at the top crimp, extending above the cup, which are, in order, case A, insulation B, cover C, insulation D, and case E layers.